Getting a "java.lang.NullPointerException" while working whith Maps

Question

I am trying to write a simple Diet program using maps with this code. But I keep getting NPE which keeps interrupting the process at random.

What is wrong with my code? Why is that that I keep getting this error and how do I fix it?

class Diet {
  public static void main(String[] args) {

    Map < Integer, String > FandVMap = new HashMap < Integer, String > (15);

    FandVMap.put(1, "A bowl of Salad");

    /* .
     .
     .
    */

    FandVMap.put(12, "A Banana");

    //************************************************************

    Map < String, Integer > CaloryMap = new HashMap < String, Integer > (30);

    CaloryMap.put("An Orange", 30);
    .
    .
    .
    CaloryMap.put("A bowl of Salad", 30);

    Random randomGenerator = new Random();
    randomGenerator = new Random();
    int i = 0;
    //int rand;
    while (true) {
      Integer rand = 0;
      rand = randomGenerator.nextInt(12);

      String name = FandVMap.get(rand);
      System.out.println(name);
      Integer Calory = 0;
      Calory = CaloryMap.get(name); // This is where the problem occurs. <<========
      int Sum=0;
      Sum=Sum+Calory.intValue();
      System.out.println(Sum);

      if (Sum > 1000) {
        break;
      }
    }
  }
}

And here is the output I get:

A Peach
50
null
A bowl of Salad
30
A Nectarine
50
java.lang.NullPointerException
at Diet.main(gadas.java:83)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)

The Calory Sum isn't working either.

Because your CaloryMap don't have value that you expect. Check that all names from FandVMap are presented in CaloryMap. — cybersoft
– cybersoft, Commented Jun 8, 2015 at 20:20
If you want to map a small ordinal integer value to a String, using HashMap is overkill. There's a structure for that, and it's called "array". — Forketyfork
– Forketyfork, Commented Jun 8, 2015 at 20:27
You will never leave the while loop because your sum will always be zeroed — ha9u63a7
– ha9u63a7, Commented Jun 8, 2015 at 20:35

Razib · Accepted Answer · 2015-06-12 06:51:39Z

6

The method randomGenerator.nextInt(int n) - returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and n (exclusive).

So I think at some point when randomGenerator.nextInt(int n) returns 0 then -

String name = FandVMap.get(rand)

returns null and hence -

Calory = CaloryMap.get(name);

also evaluted to null

Moreover if you FandVMap map doesn't contains a key from range 1 to 12, then the error may occurred for more than one case. Suppouse your FandVMap map doesn't contain a key 4 then FandVMap.get(4) also returns null.

edited Jun 12, 2015 at 6:51

answered Jun 8, 2015 at 20:22

Razib

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1 Comment

Ghaghaleelee Over a year ago

Thank you.i fixed it. i cant believe Such a simple problem had confused me for several hours. and the sum problem was even worse!. tnx.

ajacian81 · Accepted Answer · 2015-06-08 20:19:41Z

0

String name = FandVMap.get(rand);

This is very likely returning a null, which you don't check for. Also, your variable naming doesn't follow standard protocols, your variable instances should be camel cased starting with a lower case letter. For instance, CaloryMap should become caloryMap. It doesn't affect this directly but it's good to bring up.

answered Jun 8, 2015 at 20:19

ajacian81

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Comments

frhack · Accepted Answer · 2015-06-08 20:26:46Z

0

Whith randomGenerator.nextInt(12) you get a random number from 0 (inclusive) to 12 (exlusive)

so if you get 0 FandVMap.get(rand) will be null because you don't have an entry whith 0 as key in the HashMap.

rand = randomGenerator.nextInt(12);

  String name = FandVMap.get(rand);
  System.out.println(name);
  Integer Calory = 0;
  Calory = CaloryMap.get(name);

from JavaDoc: public int nextInt(int n) Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)*

answered Jun 8, 2015 at 20:26

frhack

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